Mark Phelan: Trip computers the best way to get a read on fuel economy

1:01 PM, February 2, 2014

Gas station pumps are usually accurate — they're regulated by the states — about how much fuel they deliver, but it's almost impossible for the average driver to be sure the tank is equally full from one stop to another. That's because of variables such as temperature, sloping gas station parking lots and pumps' flow rates. / Seth Perlman/Associated Press

The trip computer in a 2014 Mercedes-Benz E250 Bluetec diesel. Built-in trip computers have become the most accurate way to measure your vehicle's fuel economy. / Mark Phelan/Detroit Free Press

Gas station pumps are usually accurate about how much fuel they deliver, but it's almost impossible for the average driver to be sure the tank is equally full from one stop to another. / Scott Barbour/Getty Images

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The way many of us calculate our fuel economy is wrong, but there’s an easy way to get accurate figures for how much fuel you use.

I’ve never been more surprised than when Drew Winter, editor-in-chief of Ward’s Auto World magazine, raved to me about the fuel economy readings from the trip computer in a Chevrolet Cruze diesel he drove from Detroit to Traverse City, 300 miles away in northern Michigan.

“Trip computer” is the auto industry’s name for the little electronic readouts that show your average fuel economy, distance to empty, and often other statistics like average speed and instantaneous fuel economy. The information is usually displayed on the instrument cluster near the odometer.

Trip computers have been around since at least the 1980s, but the early ones were not very accurate, because the vehicles lacked sophisticated electronics.

I trusted them about as much as the e-mails that promise beautiful Russian women are dying to meet me. No offense, tovarisch. I’m sure you have a lovely dacha.

I calculated my fuel economy the old way, the way my Dad taught me in high school, which was probably the way he calculated if a plane could make it over the Himalayas and back to his air base in WWII. I filled the tank, drove, refilled and divided the number of miles by the amount of fuel to refill.

I might as well have been using a WWI walkie talkie in an iPhone world.

“The trip computer is absolutely the best way to track fuel economy,” said Wayne Powell, vice president of electronic systems at Toyota’s tech center in Ann Arbor. “It takes that information, compares it to how far you’ve traveled and does the math. It gets a very precise monitoring of the amount of fuel being used.”

A wide range of experts agree.

“We need to know exactly how much fuel is being used by every stroke of every cylinder,” to meet emissions and fuel economy standards, said Roger Clark, senior manager of GM’s energy center. “We do a lot of precision measuring of the fuel injectors and exhaust treatment system.” The computer converts that data into fuel economy.

Ward’s relies exclusively on trip computers for its annual list of the industry’s 10 best engines, said Tom Murphy, Ward’s executive editor.

The old method of filling at the pump and dividing miles by gallons is also less reliable than you might expect.

Gas station pumps are usually accurate — they’re regulated by the states — about how much fuel they deliver, but it’s almost impossible for the average driver to be sure the tank is equally full from one stop to another. That’s because of variables such as temperature, sloping gas station parking lots and pumps’ flow rates.

“The fuel fillers on pumps have lots of variation,” said Frank Markus, technical director of Motor Trend magazine. “It’s very unrealistic to measure fuel consumption based on the pump. Even the same pump can be way off as to what constitutes a full tank from one stop to the next.”

Experts don’t have much faith in smartphone fuel economy apps either. They’re good calculators, but don’t have direct access to the engine to determine the exact amount of fuel used.

“The car’s sources of information about fuel economy are very sophisticated,” said Brian West, of Oak Ridge National Laboratory’s fuels, engines and emissions research center.

Motor Trend pays millions of dollars for equipment that traps and measures the exhaust of the 300 or so vehicles it tests every year. Automakers and the government shell out even more for lab tests and test tracks.

For you and me, the best way to track fuel economy is probably our vehicles’ trip computers.